1. Field of the Invention
The present invention generally relates to a high-performance tape-shaped magnetic medium that allows for high-density recording in the sub micron region, and more particularly, relates to a tape-shaped magnetic medium for information recording that can be exchangeable with a data storage magnetic medium in use for digital recording.
2. Description of the Prior Art
Ever increasing capacity of various types of information requires a medium that provides high-density recording and saves space for libraries. For example, various kinds of sequential data from a satellite are stored for a long period of time after they are recorded. In such an example as this, libraries increase every year and securing a space for the storage is considerably expensive. There is also a demand for a compact and high-capacity recording medium for use as a backup of ordinary information data. Currently there are two major rapidly advancing data storage systems that meet these requirements: a digital data storage (DDS) that utilizes a mechanical system of a digital audio tape recorder (DAT) for data recording and a system that employs an 8 mm VTR mechanism.
The current major magnetic recording medium, particularly in use for a DDS and an 8 mm system, is one coated with metal magnetic powder. The metal coated medium has taken advantage of large magnetization and high coercive force that suppresses self-demagnetization and recording-demagnetization for increasing recording density. It has exhibited, however, a problem of large demagnetization during storage for a long period of time at a high temperature and high humidity causing, and in the worst case, a complete loss of recorded data. In order to increase a recording, density metal alloy films having a high saturated magnetic flux density are also used as recording magnetic films for magnetic recording heads. When this kind of magnetic head is used in the double speed data recording mode, the medium is burnt to the head resulting in considerable reduction of output signals. Therefore, aforementioned magnetic recording medium currently used widely satisfies the required electromagnetic conversion characteristics for reading data, but soils the magnetic head by being burnt to it and fails to demonstrate a good environmental storage capability. Recently hexagonal ferrite powder materials such as barium ferrite have attracted an attention for solving these problems. These powder materials are acid resistant, provide a good environmental stability and an excellent short-wavelength recording characteristic, and have an axis of easy magnetization perpendicular to a plane. A tape-shaped magnetic medium having an axis of easy magnetization aligned in the longitudinal direction is under development by using these magnetic materials. (For example, Japanese patent laid-open publication SHO 58-6525 and Japanese patent laid-open publication SHO 60-164925.) In order to increase a capacity of a tape-shaped magnetic medium an attempt to lessen the total thickness of the tape-shaped magnetic medium is being made. This method is known to weaken the mechanical strength of the tape-shaped magnetic medium such as Young's modulus and flexural rigidity, to degrade traveling performance and durability of the magnetic medium, to cause a contact of the tape-shaped magnetic medium with the magnetic head to be unstable, and accordingly to reduce the electromagnetic conversion efficiency. (For example, Japanese patent laid-open publication SHO 56-11624 and Japanese patent laid-open publication SHO 59-135632.) In order to compensate the reduction of the flexural rigidity due to the decrease of the total thickness of the tape-shaped magnetic medium attempts have been made to improve the strength of the nonmagnetic base film which is a major part in volume of the tape-like magnetic material, because the flexural rigidity is proportional to the cube of the thickness of the tape-shaped magnetic medium and also the Young's modulus. Those attempts include a replacement of a polyethylene terephthalate film (referred to as PET) commonly used for AV related products with a polyethylene naphthalate film (referred to as PEN) having higher Young's modulus (for example Japanese patent laid-open publication SHO 62-88136 and Japanese patent laid-open publication SHO 62-117137) and use of aromatic polyamide films (Japanese patent laid-open publication SHO 56-11624 and Japanese patent laid-open publication SHO 62-62424). Other attempts include laminating a base film with a metal film and mixing the magnetic powder with particles of an anisotropic shape which are aligned along the longitudinal direction of the tape-like medium. (Japanese patent laid-open publication SHO 63-118116 and Japanese patent laid-open publication HEI 2-260228). Most of them except for the PEN films are costly to implement them and they are unlikely to be used for commercial products. The flexural rigidity of the tape-shaped magnetic medium depends not only on the total thickness of the tape-shaped magnetic medium but considerably on the shapes of the magnetic powder particles and their alignment in the medium. That is, if acicular magnetic powder particles are aligned along the longitudinal direction of the tape-shaped magnetic medium, the Young's modulus increases in the same direction. In contrast, the Young's modulus for the direction perpendicular to the alignment, the transverse direction in the above example, tends to decrease. This phenomenon exhibits itself more profoundly for a larger profile ratio of the acicular magnetic powder particles (the major axis divided by the minor axis). Therefore, generally, magnetic tapes comprising acicular magnetic powder coated on a nonmagnetic base film such as PET and PEN are designed so that the magnetic layer has larger Young's modulus than that of the nonmagnetic base film in the longitudinal direction of the tape-shaped magnetic medium.
In a case where the magnetic powder particles are ferromagnetic hexagonal ferrite which has an axis of easy magnetization perpendicular to a plane, the Young's modulus in the longitudinal direction of the medium tends to decrease as the magnetic particles are aligned in the longitudinal direction of the tape-shaped magnetic medium. This decrease is enhanced with an increasing aspect ratio (particle diameter divided by particle thickness). Thus, the tape-shaped magnetic medium comprising ferromagnetic hexagonal ferrite powder particles which are aligned in the longitudinal direction of the tape-shaped magnetic medium can be produced with an excellent short-wavelength recording characteristic and guaranteed stability for long storage. However, this design principle causes difficulties in reducing the total thickness of the tape-shaped magnetic medium which is needed for increasing the capacity of the medium: reduction of the flexural rigidity in the longitudinal direction of the tape-shaped magnetic medium resulted from reduction of the Young's modulus in the longitudinal direction of the magnetic layer and the resultant reductions of the durability of traveling of the tape-shaped magnetic medium and the electromagnetic conversion characteristics. Those problems have been difficult to solve.
Thus, for a tape-shaped magnetic medium comprising a nonmagnetic base film and a magnetic layer coated on it and made of ferromagnetic hexagonal ferrite powder particles which have an axis of easy magnetization perpendicular to the plane, the conventional design of aligning acicular magnetic powder in the longitudinal direction of the medium for reducing the medium thickness does not provide either sufficient durability for traveling nor electromagnetic properties.